Devices have been known in the past for use with materials or objects that are sensitive to fluctuations in the ambient temperature. For instance, various pharmaceuticals, units of blood, or photographic chemicals must all be maintained below certain temperatures prior to use. If these types of materials are exposed to elevated temperatures for long enough periods of time, it is desirable to provide an unambiguous indication of that fact so that the materials can be disposed of or recognized as having a reduced shelf life or effectiveness. Generally, exposure for a shorter period of time at an elevated temperature may be as damaging as an extended period of time at a lower temperature.
U.S. Pat. No. 3,962,920 entitled "Reservoir Pad For Time Indicator" and U.S. Pat. No. 3,954,011 entitled "Selected Time Interval Indicating Device", the contents of both of which are incorporated herein by reference, both disclose an indicating device that includes a reservoir containing an indicating material, preferably a colored organic material, that liquefies when exposed to temperatures at or above a predetermined temperature. A wick, made of a wood pulp paper, is provided adjacent the reservoir. Prior to use, the reservoir and wick are separated by a barrier. When placed in contact with an object, such as a container of blood, the barrier is removed. If temperatures at or above a predetermined temperature are encountered, the indicating material liquefies and begins to migrate along the wick at a controlled rate. The progress of the indicating material can be observed and, if it passes a known point on the wick, it can readily be determined that the blood has been degraded and is unfit to use. If the object that the device is attached to is a container of pharmaceuticals, the pharmaceuticals may not be harmful if used, but may be rendered ineffective. Such devices have been marketed in the past by the Minnesota Mining and Manufacturing Company of St. Paul, Minn. under the trademark "MonitorMark".TM. brand indicating devices.
Time and temperature indicating devices such as these are useful in transporting or storing perishable substances as described. However, such products or substances are frequently used in a wide variety of remote or less developed areas of the world. As a consequence, the climates that the indicating devices are exposed to vary dramatically, and facilities, such as refrigerators, available to safely store and transport the objects are less reliable and accessible. It has been found that variations in ambient humidity have a direct affect on the accuracy of the indicating devices. That is, the rate of migration of the indicating material through the wick will be altered as the ambient humidity changes. Climates that have a high ambient humidity will tend to retard the progress of the indicating material along the wick and thus inaccurately indicate that the substance attached to the device is still usable. For instance, in environments having a relative humidity of at least 90%, conventional indicating devices have been shown to have an observed rate of migration that is reduced by 200-300% over the expected rate of migration. It is believed that this effect is related to the absorption of atmospheric moisture by the generally hydrophilic wicking material, such as wood pulp paper previously mentioned. The presence of the absorbed moisture tends to increase the surface tension of the fibers in the reservoir, thereby attenuating the migration of the indicating material by capillary action along the wicking member. Likewise, climates having a low ambient humidity will tend to accelerate the expected rate of migration of the indicating material, thus falsely indicating that the substance has been degraded. Either alternative is undesirable, with the former case having potentially more serious consequences.
None of the existing indicating devices have adequately addressed the problem of variations in the ambient humidity and the effect of that variation on the accuracy of the time and temperature indicating device.